Consider the medical term “leukemia“, which is familiar to the public as referring to a form of blood cancer. It’s related to the less familiar term “leukocyte“, which refers to various kinds of white blood cells. (The prefix “leuko-” is derived from Greek leukos, meaning “white”. The suffix, “-cyte” is also Greek: kytos, meaning “cell”.)

Leukocytes were originally recognized as distinct from other types of cells in the blood, especially “red” blood cells, which derive their color from iron-containing hemoglobin. There are actually a number of different types of leukocytes – and different types of corresponding leukemias. One common subtype of leukemia involves myeloid cells (myelocytes), which are normally found in bone marrow and occur as precursors to several types of blood cells. Acute myeloid leukemia (AML, also known as acute myelogenous leukemia) is the most common example, and has several subtypes itself.

Leukocytes of many types are derived from myeloid cells, which are thus a type of stem cell. When such cells develop certain types of abnormalities they harmfully overproduce derived cells, effectively making them cancer (specifically, leukemia) stem cells. The most common type of abnormality is a type of cell surface receptor known as CD123. A receptor is simply a protein found on a cell surface which binds to external cell signaling proteins called cytokines. (There’s the “cyto-” again. The “-kine” part is from Greek kinos, which refers to motion, as in “kinetic”.)

Cytokines are often interpreted by cells as signals to divide and proliferate, usually in a helpful way, as normal with immune system cells. Certain immune-system cytokines are called interleukins, because they facilitate signaling among immune system leukocytes. CD123 receptors, in particular, are receptors for interleukin-3. Thus CD123 receptors have another name: interleukin-3 receptor, alpha.

CD123 is essential for the normal communication between immune system cells such as T cells. It must exist on the surfaces of cells that need to respond to interleukin-3, in order to have a proper immune system response to infection. You do not, however, want CD123 on stem cells, whose excessive proliferation results in leukemia.

And so it is that one promising treatment for acute myeloid leukemia involves the development of a novel antibody, called 7G3, that can block CD123 receptors without triggering proliferation. Of course, that might interfere with immune system function – but such interference is preferable to leukemia.

New Targeted Therapy Finds And Eliminates Deadly Leukemia Stem Cells (7/2/09)

Associate Professor Lock [senior study author] and colleagues exploited the fact that the molecule CD123 is expressed at very high levels on LSCs but not on normal blood cells. CD123 is part of the interleukin-3 receptor, a protein that interacts with a growth factor (called a cytokine) that influences cell survival and proliferation. The researchers created a therapeutic antibody that recognized and bound to CD123 with the hope that this antibody would selectively interfere with AML-LSC survival.

When AML-LSCs from human patients were transplanted into mice treated with the antibody, called 7G3, cytokine signaling in the tumor cells was blocked. Further, 7G3 impaired migration of the AML-LSCs to bone marrow and activated the innate immune system of the host mouse to destroy the AML-LSCs. Overall, treatment with 7G3 substantially improved mouse survival when compared with control groups. The researchers go on to report that a CD123-targeting antibody is currently being used in phase 1 clinical trials of advanced AML and that there are no signs of treatment-related toxicity.

Further reading:

New Drug Hits Leukemia Early (7/2/09) – Science News article

Monoclonal Antibody-Mediated Targeting of CD123, IL-3 Receptor α Chain, Eliminates Human Acute Myeloid Leukemic Stem Cells – Cell Stem Cell research article

Tags: cancer, cancer stem cell

Nevertheless, very solid evidence has been accumulated over the years for the existence of two types of black holes: stellar-mass black holes with masses from 3 to several tens of solar masses (M_⊙), and supermassive black holes, which are vastly larger – generally millions to billions M_⊙. Concerning some of the evidence, see here.

Stellar-mass black holes are easy to explain as supernova remnants, while supermassive black holes seem to be an inseparable concomitant of the development of all galaxies.

Perhaps surprisingly, however, there has been very little evidence for the existence of black holes of intermediate mass. If such black holes exist at all, the processes that form them must be rather more unusual. Evidence for the existence of intermediate mass black holes has been reported in the past. (There’s some discussion here of possible black holes of mass less than a million M_⊙.)

But because black holes, by their nature, are difficult to observe directly, and so their existence must be inferred indirectly, it has been difficult to come up with relatively unambiguous evidence. Now we have announcements of better evidence in two cases.

New Class Of Black Holes Discovered (7/1/09)

A new class of black hole, more than 500 times the mass of the Sun, has been discovered by an international team of astronomers.

The finding in a distant galaxy approximately 290 million light years from Earth is reported today in the journal Nature.

Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the centre of galaxies, or about the size of a typical star (between three and 20 Solar masses).

The new discovery is the first solid evidence of a new class of medium-sized black holes.

Important discoveries often don’t come by themselves. Other researchers and teams tend to report related results at the same time. And this is no exception. The above reports concern a candidate object in a galaxy (ESO 243-49) about 290 million light-years away. But there’s also a report of an object much closer, in the globular cluster M54 (more here), which is only about 87,000 light-years away. It’s thought to belong, actually, not to the Milky Way itself, but rather to the Saggitarius Dwarf Elliptical Galaxy, a satellite of the Milky Way.

Density and kinematic cusps in M54 at the heart of the Sagittarius dwarf galaxy: evidence for a 10⁴ M_⊙ Black Hole?

We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line of sight velocity dispersion is 20.2 +/- 0.7 km/s, decreasing to 16.4 +/- 0.4 km/s at 2.5″ (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole (BH) mass of ~ 9400 M_⊙. However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy.

M54

Further reading (ESO 243-49 candidate object):

Finally, an Average Black Hole (7/1/09) – ScienceNOW

New Candidates for Midsize Black Holes (7/3/09) – Sky and Telescope

An intermediate-mass black hole of over 500 solar masses in the galaxy ESO – Nature research article

XMM-Newton discovers a new class of black holes (7/1/09) – ESA press release

New Observations Suggest Mid-Size Black Holes Exist (7/1/09) – Space.com

Black holes: now available in size ‘M’ (7/2/09) – Cosmos magazine

Astronomers Discover Medium-Sized Class of Black Holes (7/1/09) – Universe Today

Intermediate-mass black hole (7/1/09) – Science Centric

Astronomers sniff intermediate mass black hole (7/2/09) – The Register

Astronomers Size Up a Candidate for Midsize Black Hole (7/1/09) – Scientific American

New Class of Black Holes Discovered (7/1/09) – Wired

X-rays are smoking gun for middleweight black holes (7/1/09) – New Scientist

A New Kind of Black Hole (7/2/09) – Smithsonian.com

Further reading (M54 candidate object):

Density and kinematic cusps in M54 at the heart of the Sagittarius dwarf galaxy: evidence for a 10^4 M_sun Black Hole? – Astrophysical Journal research article

Tags: black holes

Of course, this just reflects my own interests; yours may be very different. In particular, I’m concerned here mostly only with blogs that actually have a lot of discussion of actual science. There’s a place for blogs that cover the writer’s extra-scientific interests, personal life, etc. Finding blogs with a broader scope is a different issue I’m not addressing here.

RSS feeds

I offer some of what seem to me to be the most interesting science blogs at my Bloglines page. The entries are organized into broad categories, so searching shouldn’t be too hard.

It’s not necessary to understand much about RSS. Use of the page is pretty self-explanatory. But using an RSS reader like Bloglines make scanning many blogs for new material pretty easy. Google Reader is perhaps an even better tool, but it’s not used for this list because it does not allow for making the list of feeds public.

Blog rolls

Once you have a certain number of blogs you know you like, my preferred method of finding new ones is simply by going through the “blog rolls” provided by the blogs you like best. These lists are usually presented in one of the blog’s side columns, but sometimes they may be on a separate page. Just go through the list one at a time, trying blogs you haven’t visited before (or recently).

Inevitably, many of the links will turn out to be dead ends. That will be obvious pretty quickly. But it does stand to reason that if you like the blog of the person who made the list, you will probably like some of the blogger’s choices.

I think I’ve put together a fairly good list at my main blog, Science and Reason. It’s not the same as what’s in the RSS feeds saved at Bloglines, and undoubtedly there have been blogs that have moved or disappeared since I last checked. But it’s a place to start.

Research Blogging

Research Blogging is a site hosted by Seed Media Group. It is not a single blog or group of blogs, but rather an “aggregator” that provides blog posts from many sources and all scientific fields. Posts are required to deal with research published in refereed journals or certain other professional venues. Posts may be contributed by any science blogger, not necessarily one on ScienceBlogs, as long as the stated criteria are met. This is an excellent way to scan recent research developments, as well as to become introduced to blogs you may not have been aware of.

Check here for Science and Reason posts on Research Blogging

Facebook NetworkedBlogs

If you’re a Facebook member (and who isn’t, these days?), there’s a useful application available called NetworkedBlogs. If you’re logged in to Facebook, simply go to the URL just given and select “Browse” from the menu at top, then select “science” from the menu on the right. Or better yet, just go straight to here. What you’ll get is the “Top 50 blogs in science”, ranked by number of “followers”. This gives you a selection of blogs to start with. You can click on the picture or name of any blog that looks interesting to get more information about it. That includes a description of the blog, appropriate tags (which can be searched for other blogs with that tag), recent blog posts, links to other users who are following the blog, and a list of other blogs that may be similar.

Hit the big blue “Follow” button for any you want to add to your personal list. Once you’ve followed a few blogs, you can go to your personal page (“Profile” from the menu at top) to see the list of blogs you’ve selected to follow. (Clicking the checked “Follow” button again will remove it from the list.) Also, when you return to the NetworkedBlogs home page, you’ll see recent posts from the blogs you’re following.

There are a number of other things you can do to search for additional blogs. For example, selecting “Friends” from the top menu will show you a list of recent posts from blogs belonging to friends of yours. You’ll also have the option to get a list of all your friends’ blogs, or even of blogs that your friends follow (which won’t necessarily be related to science).

If you have a blog of your own that isn’t listed yet, you can add it here, or watch for a button labelled “Add a New Blog”. There’s also a way to use NetworkedBlogs to make it easier to add new posts from your blog to your Wall (go here).

By the way, the NetworkedBlogs page for Science and Reason is here. (Also accessible here.) Please follow it, and leave a comment on the Wall you’ll find there.

All in all, this is a pretty easy way for Facebook users to browse and discover new blogs.

Science blog networks

By this I mean groups of blogs that are usually hosted at a single site and have a uniform style and appearance. The blogs are usually listed by categories at the site’s main page. Sometimes blogs are included only by invitation of the hosting organization, while others are open to just about anyone. In either case, there will be quite a lot of variation from one blog to another, so there’s no guarantee that all the blogs will be high-quality and well-maintained.

Here are a few of the networks I know about:

Blogs on Nature Network

One has to join the Nature Network (which is open to anyone) and apply to start a blog, but the process doesn’t appear to be difficult, and it’s free. The blogs in this network are high-quality and deal mostly with real science. Separately, Nature also provides a (confusingly named) blog catalog: Nature Blogs, which is described below.

Scientific Blogging

This is a commercial operation, but it appears that anyone can start a blog on a science topic for no charge. Quality is a little uneven, but generally pretty good.

ScienceBlog

The frontpage content is drawn from selected press releases about scientific research, but readers can create their own blogs. Recent blog posts from readers, as well as the reader blogs are accessed from a separate page.

ScienceBlogs

The site is provided by a commercial organization, Seed Media Group, which provides several other online services and also publishes a print magazine. Blogging at the site is by invitation only, and most of the bloggers are science professionals or graduate students. Nevertheless, the quality is somewhat uneven, as the majority of blogs tend to deal with political and cultural issues, or personal/professional anecdotes – so there can be a lot to wade through in search of material actually covering scientific research. But the best of the lot are very good, and usually quite well known.

Blog catalogs and directories

A catalog or directory consists of links and short descriptions of blogs that are actually hosted elsewhere. The directory may allow additions by anyone or only entries selected by editors. The latter, naturally, tend to have much higher quality content. However, in the better open directories, there are mechanisms for ratings by such means as traffic analysis, link popularity, reader voting, etc. There’s a large diversity in how such catalogs and directories operate. Some are specifically for science blogs, while in others science is one category among many.

There are several desirable features to look for in a blog directory. One is the inclusion of tags to describe blogs, in addition to or instead of fixed categories. This takes some of the guesswork out of finding blogs that interest you, since blogs can have multiple tags associated with them. Another important feature, or set of features, is some sort of social networking capabilities, especially the ability to make “connections” with other users and to form groups of users interested in similar things. Other users can be very helpful in identifying blogs you want to read. It’s also helpful if there is a capability for including user comments and discussions on catalog entries.

You might want to take a look at the Wikipedia Blog directory page (now covering directories in general).

Here are some of the better examples of blog catalogs:

Nature Blogs

Included in Nature’s stable of online sites is a decent blog catalog, and as you’d expect it’s specifically for science blogs. In addition to blogs maintained by Nature staff and contributors, outside blogs are also listed (upon approval). In addition to broad categories, tags can be assigned. Other convenient features are listings of recently approved blogs and blogs ranked by “popularity”. But perhaps the nicest feature is the listing of recent stories from Nature blogs and outside sources, together with commentary on them from other blog posting. This site is highly recommended.

Scintilla Sources: blogs

Scintilla is another corner of the Nature empire. It appears to be somewhat of an experiment for identifying important current science news stories by doing content analysis of a large number of sources, including blogs. The system will attempt to recommend articles from its sources that match a user’s interests. Social interest groups and connections are supported. But it is the actual list of article sources that Scintilla uses which is relevant for supplying a list of blogs to investigate. The Science and Reason blog has its own page at Scintilla.

Technorati: Science

Technorati is one of the old-timers in the blog tracking business, and it can provide a variety of kinds of blog information. One of its services is a directory, organized by category. Listed blogs are rated by “authority”, which is based on counting links to blog pages. This is, of course, not the same as blog traffic or some kind of judgment of actual quality. Technorati also provides a number of other types of search, such as by blog post tags and blogger-supplied keywords. Such searches may provide faster ways to locate blogs that deal with topics that interest you. The Science and Reason blog has its own page at Technorati, which you can add as a “favorite” if you log in to Technorati.

BlogCatalog.com: Science

BlogCatalog is a general-interest catalog that has a category specifically for science, and a few subcategories. Blogs are included upon owner’s suggestion, following review and approval. There are good social facilities including user profiles, friends, interest groups, and discussion boards. There are also flexible facilities for searching on various criteria. The Science and Reason blog has its own page at BlogCatalog, and can be added to a list of “favorites” if you have an account at BlogCatalog.

Best of the Web Blog Directory: Science

The Best of the Web Blog Directory is a general-interest catalog with a science category and a number of subcategories. Listings are determined by an editor – presumably not all submissions are accepted, so there is some bias towards quality. Short descriptions are provided. There are no social networking capabilities.

Blogged: Science

Blogged is a general-interest catalog with a science category and a number of subcategories. Blogs selected for inclusion are also rated by editors on a number of criteria, including “Frequency of Updates, Relevance of Content, Site Design, and Writing Style”. Listings are ordered with highest-rated sites first. Registered users of the site can add reviews of individual blogs. There are some social networking capabilities. The Science and Reason blog has its own page at Blogged, and you can “follow” it if you have an account at Blogged.

Bookmarking sites

Bookmarking sites are more informal than catalog sites. Any registered user can save bookmarks at the site, and optionally make bookmarks public, which is when they become useful to others. (Sites that don’t provide public bookmarks aren’t considered here.)

Most bookmarking sites allow for bookmarking any type of page, not just blogs and blog pages. Usually there is no way to limit browsing and searching only to blogs, unless something has been specifically tagged as a blog. All current bookmarking sites support tagging, so that one can search for general or specific pages using appropriate tags. Ordinarily it is specific articles within a blog that are bookmarked, although sometimes blog home pages will be bookmarked. There are some quite good bookmarking sites oriented towards academic users, such as Connotea, CiteULike, and BibSonomy. However, these are used mostly for bookmarking journal papers and other academic material.

Probably the best way to use bookmarking sites to find blogs is to search for fairly specific tags related to scientific topics, such as “quantum computing”. Most results will involve specific pages – examination of those results should lead you to appropriate blogs.

Most bookmarking sites now have basic or extensive social networking tools, which can also be helpful. I have a table of some of the leading bookmarking sites here. Among those, the following may be particularly useful:

Twine

Twine is a fairly untraditional bookmarking site with several attractive features. Public bookmarks are stored in “twines”, which are topical areas, usually somewhat narrow in scope. Tags are associated with twines rather than individual items. The system analyzes user choices in order to make recommendations on other items or twines a user might like. Users can “connect” to other users, but there are no special interest groups other than the twines themselves.

Delicious

Delicious is one of the oldest and most popular bookmarking sites. It has especially good support for tags associated with bookmarked items, in that you can search for items that have all of a specified set of tags. You can also group set of items with related tags into “bundles”. All search results can be accessed with RSS feeds. On the other hand, the social networking facilities are somewhat weak – no groups or detailed user profiles, for instance.

Diigo

Diigo is a relatively new bookmarking tool that has a fairly large number of features. For instance, you can call up a list of sites you have saved bookmarks from, and view recent additions to the site. You can also get a list of public bookmarks on the site that other users have saved. (The page for the Science and Reason blog is here.) Bookmarked items can have associated tags and also be grouped into “lists”. Diigo has extensive social networking features, including special interest groups. (There is a special interest group for the Science and Reason Network, which you are invited to join.)

StumbleUpon

StumbleUpon is one of the newer breed of “Web 2.0″ bookmarking tools with extensive social networking facilities. Registered users submit site links to sites or pages (usually the latter) that they especially like. The service allows search based on tags that are specified along with the submission. Not all submissions are blogs or blog pages, of course. Users can provide reviews or simply vote to “like” an item, so there is plenty of feedback on item quality. You can also view pages and sites at random to provide feedback on what your interests are. There are many special interest groups, including a lot for science and technology.

Blog search engines

Blog search engines are much like ordinary Web search engines, except they restrict their searches to separate blog posts rather than entire pages. That’s important, and not as trivial as it may sound. It’s not always clear (especially to a search engine) what is or isn’t a blog – and that’s important if blogs are what you’re looking for.

Blog search engines know how to recognize blogs, and find individual posts rather than whole pages. Yet searches can be done in the usual way for posts that contain specific words. More advanced searches can be done for posts that contain all of a set of words, or some words but not others, or specific phrases.

If you’re looking simply for blogs that deal with areas of interest to you, use searches to find terms or phrases that are especially likely to be used.

Technorati

Technorati is especially good for searching blogs because it keeps track of blogs that attach explicit tags to their posts. So that in addition to performing simple or complex searches for posts containing specific terms, searches can also be done on just the tags that were attached to posts. You can also do a search for other blogs that link to a specified blog.

Google Blog Search

A Google blog search is just like a regular Google search except for being restricted to blog posts instead of arbitrary pages. In particular, the usual types of advanced searching are available. The blog search home pages allows you to get a default set of top stories currently on blogs, and in various specific categories, including science. RSS feeds for blog searches are readily available (unlike in ordinary search, where it’s harder).

Icerocket Blog Search

Icerocket has been around quite a long time – even before Google took on blogs. It does blog searches well, and other kinds of searches besides – including general Web pages, news, images, and MySpace. It has also kept up (somewhat) with the times, and now does Twitter searches too. Best of all, you can quickly switch to a different kind of search if you’re not satisfied with what you’ve found already. One more nice feature is that you can turn most searches into RSS feeds, for use anytime in the future.

BlogPulse

The search capabilities of Nielsen BuzzMetrics’ BlogPulse are somewhat more limited than other blog search engines. And since the purpose of the search engine is oriented towards marketing concerns, its coverage of specialized blogs with relatively low traffic – like most science blogs – is not especially complete. But is has one interesting feature, which is the ability (called “trend results”) to display in graphical form the frequency over time of new results in particular searches. A surge of results in a period of a few days, for example, might suggest that some interesting news came out in that time frame.

• MIT
  1. 8.04 Quantum Physics I – Spring 2006, lecture notes
  2. 8.05 Quantum Physics II – Fall 2004
  3. 8.06 Quantum Physics III – Spring 2005
  4. 5.73 Introductory Quantum Mechanics I – Fall 2005 – lecture notes – writeup at EduChoices
  5. 5.74 Introductory Quantum Mechanics II – Spring 2007 – lecture notes – writeup at EduChoices
• Utah State University
  1. PHYS 6210 – Quantum Mechanics – Spring 2007, lecture notes – writeup at EduChoices

Textbooks

1. Gasiorowicz – Quantum Physics, Third Edition
2. French, Taylor – Introduction to Quantum Physics
3. Feynman, Leighton, Sands – The Feynman Lectures on Physics, Volume 3: Quantum Mechanics
4. Liboff – Introductory Quantum Mechanics (4th Edition)
5. Eisberg, Resnick – Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles
6. Griffiths – Introduction to Quantum Mechanics. 2nd ed.
7. Ohanian – Principles of Quantum Mechanics
8. Shankar – Principles of Quantum Mechanics. 2nd ed.
9. Cohen-Tannoudji – Quantum Mechanics
10. Sakurai – Modern Quantum Mechanics
11. Robinett – Quantum Mechanics: Classical Results, Modern Systems, and Visualized Examples
12. Winter – Quantum Physics (2nd edition)

Useful resources

1. MERLOT resources for quantum mechanics
2. PhysiX – wiki providing material on quantum physics (atomic and molecular orbitals)
3. Wikipedia: quantum mechanics – main article
4. Wikipedia category directory: quantum mechanics
5. Wikiversity: Making sense of quantum mechanics
6. The Teaching of Quantum Mechanics – tips and ideas for teachers of quantum mechanics
7. Imperial College: Research: Quantum Information Theory – tutotials at varying levels of difficulty

Amazon “So you’d like to…”

1. Learn quantum mechanics
2. Learn Quantum Mechanics Via Worked Problems and Solutions!
3. Learn basic quantum mechanics as a chemist
4. see my bookshelf on Q.M & Particle Physics (part 1)
5. see my popular physics bookshelf (part 2)
6. see my bookshelf on Physics & Q.M. (part 3)

Amazon Listmania

1. Strange World of the Quantum – David Darling
2. Master Quantum Theory – David McMahon
3. on Quantum Theories
4. Quantum Mechanics books
5. The Best (and Worst) of Quantum Mechanics
6. Quantum Mechanics & Applications
7. Key books on Quantum Computation
8. Quantum Optics, Quantum Information: Essential Books List
9. Quantum Computation and Quantum Information
10. Quantum Information

Blogs

1. Shtetl-Optimized – by Scott Aaronson – emphasis on quantum computing – great tutorials
2. The Quantum Pontiff – by Dave Bacon – good blogroll
3. Quantum Moxie – by Ian Durham – good blogroll

Web sites

1. Quantiki – portal and wiki for quantum information theory – includes a good Introduction to Quantum Theory

Bookmarks

1. Delicious

One of the best features of Delicious is the ability to assign tags to any bookmark and to display bookmarks in groups by specific tags. Bookmarks, grouped by tag, may also be shared with other Delicious users, or anyone on the Web, via either a browser or an RSS feed.

Definitions of features:

Friends

Capability for two users of the service to “connect” to each other for purposes of sharing personal information.

Groups

Capability for service users to create named “groups” of users (who are not necessarily “friends”). A group usually has dedicated space for comments, discussions, and other kinds of information not shared with non-members. Membership may be either open or subject to approval.

Tags

A “tag” is a label that can be associated with bookmarks (and possiblity other things). It describes some attribute of the object. Tags are sometimes restricted to single words, but better implementations allow multi-word phrases.

Comments

Bookmarks are assumed to have (optional) brief descriptive text associated with them. “Comments” are an additional feature by which any user who has access to the bookmark can add further information about either the bookmark or other comments.

Multimedia

A bookmark may optionally be associated with relevant multimedia content, such as images, screen captures, sound files, videos, etc.

RSS feeds

Capability to associate an RSS feed with a list of bookmarks. The lists may represent bookmarks of one user, comments on a bookmark, bookmarks with specific tags, bookmarks of a group, etc. The more options the better.

Notes:

1. Additional Delicious features: Tags can be grouped into related “bundles”; good RSS support for tags. Negatives: tags are single-word only

2. Additional Twine features: A “twine” is a group of related bookmarks that can be private or shared; other users can add to and comment on public twines; “semantic web” features; no export, can import from several services and formats

3. Additional Diigo features: rich social structure (“communities” for specific sites or tags); watch lists (for new bookmarks for specific sites or tags, or from specific users); import/export several services and formats

4. Additional Connotea features: rich bookmark descriptions; wiki for user info (“community pages”) and general discussion; ability to place content widgets on other sites; import/export many formats (no services)

5. Additional CiteULike features: user blogs; ability to store PDF files; “watchlists” for topics of interest; related links can be associated with bookmarks; rich bookmark descriptions. Negatives: tags are single-word only

6. Additional Zigtag features: has “semantic search” for bookmarks; allows for user discussion of tag meaning; import/export capabilities for bookmarks; “semantic web” features

7. Additional Simpy features: ability to import from browser and Delicious; watch lists; ability to select groups to share tags with

8. Additional Spurl features: import/export HTML format; uses hierarchical folder structure instead of tags

9. Additional Stumbleupon features: users can vote up/down on bookmarked items. Negatives: RSS feeds are limited (no tags).